[electrogeek]

Hi everyone,

I’ve attached a mass spectrum of 4-phenylbenzoic acid that I’ve made recently, and I’m struggling to assign a couple of the major peaks.

I know the peak at 198 is due to the molecule itself as an ion (the sample was dissolved in methanol). I also know that the peak at 181 is due to a molecular ion of the compound without the hydroxyl part which is lost as a fragment.

But I’m struggling with the peak at 152 - at first, I thought it would be the biphenyl ion fragment, but that gives an m/z of 153, and not 152. Likewise, I thought the one at 76 would be the phenyl ion fragment, but shouldn’t that be at 77, and not 76. I also considered a +2 charge, but these values would be much lower and so I have ignored them. I’ve looked at other mass spectra of the same compound and they also have major peaks at 152 and 76. So I was wondering why it seems that the peak at 152 is [C₁₂H₈]⁺, and not [C₁₂H₉]⁺?

I also don't understand why I get two peaks for the GC (also attached), even though the mass spectra for both of the peaks are the same - the only difference I can see is a tiny peak at 227 for line one, which is absent for the other peak. We haven't done any theory on GC yet so any resources that would point me in the right direction for this will be greatly appreciated!

Thanks

Edit: No worries - worked it out!